Comparison of Different Numbers of White Base Coat Layers on Metallized Cardboard for Obtaining High Print Quality After Rubbing

Abstract

Metallized papers or cardboards, used when high barrier properties are required in packaging, are usually coated with white ink prior to printing to ensure accurate colors and high print quality. The coating provides well-controlled sorption properties at a certain thickness, allowing for better printability and reduced penetration of ink components into the substrate. The white ink used for coating ensures the dimensional stability of the substrate after the drying process is complete. This research compares how different numbers of white base coat layers affect the print quality of multicolor offset prints onto metallized cardboard after rubbing. A high print quality assessment after rubbing was obtained based on spectrophotometric and gloss measurements. A comparison of the number of white base coat layers on metallized cardboard indicated that multicolor prints with two base coat layers have lower reflectance, better color stability, and high print quality after rubbing. Gloss measurements showed that prints with one layer of white base coat exhibited higher gloss values, while rubbing led to a moderate increase in gloss for all samples. Ultimately, a thicker layer of white base coat enhances mechanical resistance while maintaining acceptable optical properties in multicolor prints on metallized cardboards.

1. Introduction

In today’s market-oriented world, packaging no longer serves only to protect the product and provide basic information; it also acts as a means of communication between a brand and its customers. Packaging design, color reproduction, and the overall visual experience play a crucial role in how end users perceive product quality. Besides color reproduction, surface gloss is also an important visual parameter that contributes to the overall appearance and perceived quality of packaging. This is especially important for products aiming to be seen as luxurious, and to attract customers. For this reason, metallized cardboard (cardboard coated with a reflective aluminum layer) is increasingly used in packaging, as its striking visual effect creates a sense of high quality for customers.

However, printing on metallized substrates presents challenges due to the optical properties of these materials. With transparent and semi-transparent inks, light can reflect off the metallized surface, resulting in color distortions, reduced contrast, and diminished legibility. To better control the optical properties of such substrates and achieve satisfactory print quality, a white base coat is typically applied. This base coat neutralizes reflectance from the metallized surface and enables higher print quality.

The number of white base coat layers can significantly affect the optical and visual properties of the final product. For example, surfaces covered with a solid white base coat behave similarly to conventional white paper, while unprinted areas can be used to achieve a metallic effect. This practice is already common in the packaging industry, and manufacturers of metallized paperboards report that varying the number of white base coat layers allows finer control of surface gloss and the combination of standard color prints with metallic effects [1]. Because the application of a white base coat layer significantly increases the total cost of printing [2], researching the optimal number of layers of white coating is very important for achieving a balance between the visual quality of the print and the economic profitability of the process [3].

Another important aspect of print quality is durability, specifically mechanical resistance (i.e., rub resistance). This property is crucial for packaging during transport, handling, and displaying at the point of sale. Printed packaging can be damaged by mechanical and environmental actions, leading to color loss, blurring, smudging, or scratches that spoil its appearance [4,5]. Research shows that a print’s resistance to rubbing depends on the internal cohesion of the layer, adhesion to the substrate, and the condition of the substrate surface, such as its roughness and porosity [6,7].

Research on paper and cardboard coatings primarily focuses on how coating composition affects print quality and durability, showing that print density depends on the interaction between the coating and paper substrate, with gloss decreasing as more coating penetrates the paper structure [8]. Other studies suggest that print quality, light scattering, surface energy, gloss, and roughness can be modified by varying the amount of binder in the coating [9,10]. Additionally, the type of coating nanoparticle (SiO₂ or TiO₂) can affect abrasion resistance, brightness, and the water contact angle [11]. Research on how the number of coating layers influences print is limited. Some findings provide insight into the effects of layering coatings of different compositions on the barrier and the mechanical properties of the paper surface, indicating slightly increased color changes with two layers of coating [12]. Other research suggests that adding up to three layers of coating can reduce the emission and retention of solvents in the ink [13]. Extensive research on how the base coat layer influences print stability was not found.

The novelty of this research is finding how many layers of white base coat ink on metallized cardboard are sufficient to improve the quality and stability of offset prints used for luxury product packaging, where these properties are extremely important.

In this work, the influence of one versus two layers of white base coat on the colorimetric components L*a*b* and their changes after rubbing on metallized cardboard are compared. Such analysis can provide insights to help the industry determine the appropriate number of white base coat layers to balance quality color reproduction and print resistance.

2. Materials and Methods

Figure 1 shows a schematic diagram of the research conducted.

Figure 1. Schematic diagram of the research.

TopFold GC2 paperboard (PT Riau Paperboard International, Riau, Indonesia) was metallized using Ultralen foil (Ultralen Film GmbH, Weil am Rhein, Germany) and the surface properties of the obtained metallized cardboards were characterized. The surface energies were determined using goniometer DataPhysics OCA 20 manufactured by DataPhysics Instruments GmbH (Filderstadt, Germany), and the geometric mean value of the dispersive and polar components of the surface tension of the test liquids were found according to the Owens, Wendt, Rabel, and Kaelble (OWRK) model [14]. As test liquids we used: glycerol (M_w = 92.10 g/mol, pro analysis, 99.5%) was supplied by Gram-Mol d.o.o. (Sveta Nedelja, Croatia), while diiodomethane (M_w = 267.84 g/mol, ≥99.4% stabilized, AnalaR NORMAPUR® analytical reagent, for mineralogy) and water (M_w = 18.02 g/mol, AnalaR NORMAPUR® ISO 3696 Grade 3, analytical reagent) were supplied by VWR BDH Prolabo (Vienna, Austria).

The roughness parameters (R_a) of the metallized cardboards’ surfaces were measured with a tactile MarSurf PS 10 profilometer manufactured by Mahr GmbH (Göttingen, Germany) by moving 4.0 mm (the evaluation length) across the surface with a measurement force of 7.5 × 10⁻⁴ N. This parameter represents the arithmetic average of the profile’s deviation.

The thickness measurements of the metallized cardboards were taken with Enrico Toniolo S.R.L. (Milano, Italy) digital electronic thickness gauge, where a cylindrical stainless steel weight creates a pressure of 0.5 kg/cm² (49.03 kPa).

The prints on metallized cardboard with one or two base coat layers of UV-curable white ink, on which the test was carried out, were printed using the offset printing technique. FD LED EU S5 printing process inks and Flash Dry white ink as base coat, from the same manufacturer, Toyo ink SDN BHD (Toyo ink Europe, Oissel, France), were used. UV-curable white ink is mainly composed of UV-curable pigment, acrylic resin, photo-initiator, diluent, and additives [15]. Photo-initiators and other ink components cure very quickly under UV radiation in a very short time immediately after application [16]. Printing was performed on a KBA RA 106 X-7 + L RS press manufactured by Koenig & Bauer AG (Würzburg, Germany). By printing full tones of two or three process inks, the tones of red (R = M + Y), green (G = C + Y), blue (B = C + M), and brown (S = C + M + Y) were achieved. A list of the samples is shown in the Table 1.

Table 1. List of the samples.

Sample Mark	Number of White Base Coat Layers	Ink Combination of Multicolor Print	Rubbing
R1	1	magenta + yellow	Before
G1		cyan + yellow
B1		cyan + magenta
S1		cyan + magenta + yellow
R2	2	magenta + yellow
G2		cyan + yellow
B2		cyan + magenta
S2		cyan + magenta + yellow
R1*	1	magenta + yellow	After
G1*		cyan + yellow
B1*		cyan + magenta
S1*		cyan + magenta + yellow
R2*	2	magenta + yellow
G2*		cyan + yellow
B2*		cyan + magenta
S2*		cyan + magenta + yellow

After printing, the prints were subjected to L*a*b* value measurement using the Ocean SR Miniature Spectrometer spectrophotometer (D50/2°) manufactured by Ocean Insight (Orlando, FL, USA), followed by rubbing tests on the Hanatek Rub and Abrasion Tester manufactured by Rhopoint Instruments (St. Leonards-on-Sea, East Sussex, UK) at 60 revolutions using a 1-pound load. Re-measuring the L*a*b* values after performing the rubbing test was done to determine the colorimetric difference (ΔE₀₀), which would provide insight into the change in print quality due to rubbing. At the same time, the reflectance spectra of the prints before and after rubbing were determined using the spherical spectrophotometer. ΔE₀₀ was calculated using Equation (1):

$$\Delta E_{00}=\sqrt{{\left(\frac{\Delta {\mathit{L}}^{\prime }}{k_{\mathrm{L}}{S}_{\mathrm{L}}}\right)}^2+{\left(\frac{\Delta {\mathit{C}}^{\prime }}{k_{\mathrm{C}}{S}_{\mathrm{C}}}\right)}^2+{\left(\frac{\Delta {\mathit{H}}^{\prime }}{k_{\mathrm{H}}{S}_{\mathrm{H}}}\right)}^2+R_{T\mathrm{}}\left(\frac{\Delta {\mathit{C}}^{\prime }}{k_{\mathrm{C}}{S}_{\mathrm{C}}}\right)\left(\frac{\Delta {\mathit{H}}^{\prime }}{k_{\mathrm{H}}{S}_{\mathrm{H}}}\right)}$$

(1)

where ΔL′ represents the transformed lightness difference; ΔC′ represents the transformed chroma difference; ΔH′ represents the transformed hue difference; R_T is the rotation function; k_L, k_C, and k_H represent the factors for the variation in the experimental conditions; and S_L, S_C, and S_H are the weighting functions.

Gloss values were additionally observed on prints before and after the rubbing test. The gloss of the prints was measured using an Elcometer 480 manufactured by Elcometer Limited (Manchester, UK) according to standards ISO 2813:2014 and DIN 67530 at a 60° angle, with an observation area of 8 mm × 16 mm and a repeatability of ±0.2 GU [17,18].

3. Results and Discussion

3.1. Metallized Cardboard Properties

The metallized cardboard (MC) used in the research with one (MC1) and two coat layers (MC2) and without a coat layer of white ink (MC0) was characterized by its surface properties (surface free energy (σ_s) and surface roughness parameter R_a), the results of which are presented in Table 2. In samples with the addition of one or two white base coat layers, there is a slight increase in total surface free energy. The increase occurs in the dispersive components of the surface free energy. The sample with two layers of white base coat has the highest surface free energy (σ_{s MC2} = 41.00 mNm⁻¹), which is almost entirely (99.85%) due to the dispersive component. Additionally, adding white base coat layers increases roughness (14.52% for MC1 and 25.81% for MC2), so the sample with two layers of white base coat has the highest R_a value (R_aMC2 = 0.777 μm).

Table 2. Surface properties of metallized cardboards.

Metallized Cardboards	Surface Free Energy (mNm−1)	Dispersion Components (mNm−1)	Polar Component (mNm−1)	Thickness (mm)	Ra (μm)
MC0	39.04 ± 1.84	35.47 ± 2.14	3.57 ± 1.73	0.406 ± 0.003	0.618 ± 0.056
MC1	39.60 ± 1.02	39.34 ± 0.98	0.26 ± 0.14	0.410 ± 0.002	0.710 ± 0.057
MC2	41.00 ± 0.82	40.94 ± 0.86	0.06 ± 0.14	0.403 ± 0.003	0.777 ± 0.077

The effect applying white ink as a base coat in one or two layers on the surface roughness of the MC0 substrate is presented in Figure 2.

Figure 2. Surface morphology diagrams of (a) MC0, (b) MC1, and (c) MC2.

3.2. L*a*b* Values Measurement

The values of colorimetric components L*a*b* were measured using a spherical spectrophotometer, and the results are shown in Figure 3, Figure 4 and Figure 5. The L*a*b* system describes color in three-dimensional space, where L* represents lightness, the a* axis shows the relationship between green and red, and b* between blue and yellow.

Figure 3. Measurement values of L* component.

Figure 4. Measurement values of a* color component.

Figure 5. Measurement values of b* color component.

Figure 3 shows the L* values of all multicolor prints before and after performing the rubbing test. It is visible that the L* value is higher for samples printed on one white base coat layer regardless of whether the samples were subjected to the rubbing test or not. Samples printed on one white base coat layer have a drop in the L* component value after rubbing for all multicolor prints (1.76% for R, 1.39% for G, 3.43% for B, and 7.80 for S) compared to unrubbed samples. For samples printed on two white base coat layers, a slight increase in the L* component after the rubbing test is visible for samples R, G, and B (0.74% for R, 0.69% for G, and 0.09% for B), while for sample S, a decrease in value (1.23%) of this color component is visible. The higher L* values for samples printed on one white base coat layer compared to those with two substrates are probably a consequence of a lower total ink application, i.e., the lower optical density of the layer. In samples printed on two white base coat layers, the layer is thicker, and more light is absorbed within the layers, resulting in lower reflectance and thus lower L* values. Such behavior of multicolor prints is consistent with the Kubelka–Munk theory, which describes the relationship between light absorption and scattering in layered ink systems: a larger layer thickness and a higher absorption coefficient led to a decrease in the overall reflectance, and thus to a darker visual impression [19].

Figure 4 shows the values of the a* color component of the L*a*b* color space on the green–red axis, where negative values of the component indicate a shift toward green, while positive values converge toward red. Positive values were recorded for R and B prints, while negative values are characteristic of G samples, which corresponds to the direction of the a* axis (green–red). Sample S has values of the component a* close to zero, which indicates that it is an almost neutral black color without a pronounced shift toward the red or green region of the spectrum. Samples with two underlying colors have higher absolute values of component a* compared to samples with one underlying color (4.44% before and 1.64% after rubbing for R, 1.71% before and 1.45% after rubbing for G, 21.32% before and 12.53% after rubbing for B, and 37.31% after rubbing for S), which indicates higher color saturation, that is, more pronounced red and green tones. The only exception is the S sample before rubbing, where a decrease (55.39%) in the green tone values occurs in the sample with an additional white layer. This effect may be due to the reduction of optical mixing with the substrate, as the additional white layer enhances the chromatic purity of the print. In samples measured after rubbing, there were no significant changes in the values of the a* component, but a slight decrease in its value was observed in most samples, which may indicate a slight loss of pigment and lower saturation after mechanical action.

Figure 5 shows the values from measuring the b* color component of the L*a*b* color space. The b* color component in the L*a*b* space is defined as the position of a color on the yellow–blue axis, with a shift toward yellow having a positive value and a shift toward blue having a negative value. Positive values for the b* color component were observed in R, G, and S prints, indicating the presence of a warm, or yellowish, undertone. On the other hand, B prints, as expected, show negative values for the b* color component, which is consistent with their spectral characteristic of a shift toward the blue part of the spectrum. Samples with two white base coat layers show slightly more pronounced b* values compared to samples with one white base coat layer (2.15% before and 8.16% after rubbing for R, 10.00% before and 18.11% after rubbing for G, 25.57% before and 5.50% after rubbing for B, and 31.35% after rubbing for S), indicating a higher saturation of chromatic tones. Again, the only exception is the S sample before rubbing, in which a decrease (5.96%) in the yellow tone values occurs in the sample with an additional white layer.

3.3. ΔE₀₀ Calculation

The ΔE₀₀ values for all multicolor prints subjected to rubbing are shown in Figure 6. The results show the total colorimetric difference of the samples before and after rubbing. It is clearly noticeable that in almost all prints, the total colorimetric difference is greater for prints printed on one white base coat layer compared to those printed on two white base coat layers (1.74 for R1-R1* and 0.28 for R2-R2*, 2.55 for B1-B1* and 1.32 for B2-B2*, and 3.50 for S1-S1* and 1.03 for S2-S2*). The exception to this behavior is the G print (1.15 for G1-G1* and 1.03 for G2-G2*), but it should be noted that the green print made on two white base coat layers has the highest standard deviation of ΔE₀₀ values compared to all other multicolor samples. Such results indicate a better resistance to mechanical action in prints with a thicker layer of white base coat. This highlights the importance of properly controlling the application of the white base coat layer to obtain more permanent prints. These results are also consistent with the surface free energy findings, where samples with two layers of white base coat showed higher surface free energy, indicating improved adhesion and cohesion properties, which result in better rubbing resistance.

Figure 6. ΔE₀₀ calculation results.

3.4. Reflectance Spectra

Figure 7 shows the reflectance of the red (R) prints, and it can be seen that the prints, despite the rubbing test that was performed and the difference in the number of layers of white base coat applied on the metallized cardboard before printing, show a stable and pronounced reflectance in the red part of the spectrum, with a maximum between 600 nm and 620 nm. This also confirms the dominant reflectance in the red part of the spectrum. Additional changes in reflectance were not recorded, meaning that the rubbing test did not have a major impact on the reflectance spectrum of red prints. It can be concluded that a double white base coat layer applied on metallized cardboard before printing contributes to better color stability during mechanical action compared to prints made on metallized cardboard substrate with a single white base coat layer, as no differences in the reflectance spectra were observed with this substrate.

Figure 7. Reflectance spectra of red prints.

Figure 8 shows the reflectance spectra for the green (G) prints. For all the prints, there is a specific pronounced reflectance in the range from 520 nm to 540 nm, which is characteristic of the color itself. Prints made on metallized cardboard with one white layer (G1) have higher reflectance values than prints made on metallized cardboard substrates with two white base coat layers (G2). After the rubbing test, the reflectance of the G1* sample shows only a minimal decrease in intensity, which indicates a good persistence of the green print. In the case of samples with two white base coat layers, the reflectance is slightly lower in the entire spectrum, and the differences before (G2) and after rubbing (G2*) are minimal. The double white base coat layer does not increase the reflectance, but it certainly contributes to the stability of the green print during rubbing.

Figure 8. Reflectance spectra of green prints.

Figure 9 shows the reflectance spectra of blue (B) prints. With a maximum between 420 nm and 470 nm, all samples show a typical spectrum expected for blue color. In this region, the highest reflectance is observed for sample B1* (blue print on substrate with one white base coat layer after the rubbing test), which can be associated with the partial removal of pigment and the increased contribution of the white base coat layer reflectance. At a wavelength of about 500 nm, a slight change occurs, and the reflectance of the unrubbed sample increases compared to the rubbed one for prints made on substrates with one white base coat layer. At 550 nm, this difference increases even more. Despite this, the shape of the spectrum remains unchanged, indicating the stability of the blue shade. Both before (B2) and after the rubbing test (B2*), prints made on a substrate with two white base coat layers show lower reflectance values compared to prints on substrates with a single white base coat layer. In the initial part of the spectrum, the reflectance of sample B2* is slightly higher than B2, while at higher wavelengths their curves almost completely overlap.

Figure 9. Reflectance spectra of blue prints.

Figure 10 shows the reflectance spectra of brown (S) prints obtained by combining cyan, magenta, and yellow ink. All brown prints show a characteristic spectral shape specific to darker tones, with lower reflectance and a minimum between 550 nm and 600 nm. Both before (S1) and after rubbing (S1*), prints made on substrate with one white base coat layer have higher reflectance values compared to prints made on substrate with two white base coat layers (S2). At a wavelength of around 450 nm, there is a decrease in reflectance for sample S1* compared to sample S1, but its reflectance is still higher compared to samples S2 and S2*. Samples made on substrate with two white base coat layers have very similar reflectance values both before (S2) and after rubbing (S2*).

Figure 10. Reflectance spectra of brown prints.

From the reflectance spectra, it can be seen that for all four multicolor prints (R, G, B and S), the reflectance is higher for prints made on metallized carboard with one white base coat layer, both before (R1, G1, B1, and S1) and after the rubbing test (R1*, G1*, B1*, and S1*). Such results can be explained by the higher scattering and partial absorption within the thicker layer of the white base coat layer in multicolor prints made on substrates with two white base coat layers (R2, G2, B2, and S2). The samples with two white layers also exhibit slightly higher surface roughness (R_a), which contributes to additional surface-level scattering and further reduces the amount of light returning to the detector. The white base coat layer printing ink often contains fine particles with a high refractive index, such as TiO₂ dispersed within a polymer binder. These particles function as light scatterers, which can be explained by the Mie scattering theory [20], according to which the scattering intensity depends on the size of the particles in relation to the wavelength, their concentration, and the layer thickness [21]. In a thicker layer, light takes a longer path through an area full of particles, which causes the rays to be refracted and scattered multiple times while part of the light is directed deeper into the layer, and only a smaller part returns to the surface, resulting in a higher loss of reflected light due to multiple scattering within the layer [22].

3.5. Gloss Values

In Figure 11, all multicolor samples printed with one coat of white have higher gloss (20.14–23.44 GU) compared to multicolor samples with two coats of white (9.58–11.17 GU). All samples show an increase in gloss values after rubbing. The most noticeable increase in gloss is observed in the blue prints B1 (12.04%) and B2 (7.84%) containing C+M, with the most significant increase in the print with one coat of white (B1). In other multicolor prints where yellow is the last ink (R, G, and S), gloss remains more stable after rubbing.

Figure 11. Gloss unit values of the prints before and after rubbing.

4. Conclusions

The comparison of different numbers of white base coat layers on metallized cardboard after rubbing was conducted to suggest a pathway for achieving high quality and stability in offset prints used in luxury product packaging.

Prints with two base coat layers have lower reflectance, better color stability, and high print quality after rubbing. This type of double layer coating enhances light scattering, while the higher free surface energy improves the adhesion of the inks.

Gloss measurements showed that prints with one layer of white base coat exhibited higher gloss values, and rubbing led to a moderate increase in gloss for all samples.

Ultimately, this confirms that a thicker layer of white base coat enhances mechanical resistance while maintaining acceptable optical properties in multicolor prints on metallized cardboards.